The proposed experiments represent the first steps in a projected research program designed to specify the neuroendocrine mechanisms and neural circuits controlling ultrasonic social communication. The specific hypotheses to be tested include: (a) The production of and rsponsiveness to ultrasounds by female hamsters depends upon ovarian estrogen, either alone or in combination with progesterone; (b) ultrasound production and responsiveness depend upon the integrity of the mesencephalic central grey area of the brain; and, (c) the dependence of ultrasonic communication on the central grey stems from a dependence of neural circuits within the central grey on activation by estrogen. The first of these hypotheses (a) will be tested by measuring rates of call production and levels of responsiveness to stimulus ultrasounds in ovariectomized females treated with a variety of gonadal steroid hormones. Hypothesis (b) will be tested by comparing ultrasound production and responsiveness in females subjected to lesions of the mesencephalic central grey versus females subjected to a control (sham) operation. The last hypothesis (c) will be evaluated by observing ultrasonic communication in in females with estradiol (versus cholesterol) implanted directly in the mesencephalic central grey.